The present invention relates generally to a method of splitting of carbon dioxide using a thermochemical cycle and, more particularly, to a method of splitting carbon dioxide using ferrite and ceria compositions in the thermochemical cycle.
CO is one fundamental component, the other being H2, of syngas, the key intermediate for synfuel production. Reactions of syngas to form hydrocarbons are thermodynamically downhill. Hydrogen can be produced renewably with commercially available technologies, for example via photovoltaic (PV)-driven electrolysis. A reasonable starting estimate for the solar-to-hydrogen efficiency is about 9% (0.12(PV)×0.75(electrolysis)=0.09). Hydrogen can then be reacted with CO2 to directly produce methanol, or indirectly to produce CO and then methanol, for example. Many of the important reactions of CO2 and H2 are not thermodynamically favorable (defined here as having a negative Gibbs free energy of reaction). (For example, the reverse water gas shift reaction is favorable only at very high temperatures and the direct synthesis of methanol is favorable only at temperatures lower than those required to carry out the conversion.) Nonetheless, it has been calculated that current technology would allow hydrocarbons to be manufactured from CO2 and electrolytic H2 with an electrical to hydrocarbon efficiency of roughly 40-50%. Thus a 5% sunlight-to-fuel efficiency is plausible for a PV-driven fuel production process.
Thermochemical cycles for water splitting are under development and avoid the efficiency-sapping sunlight to electrical energy conversion require for electrolysis and may somewhat improve the overall efficiency of both hydrogen and subsequent hydrocarbon production. Additionally, at high temperatures, CO2 is thermodynamically less stable than H2O. Thus, thermochemically splitting CO2 in a process analogous to water splitting is thermodynamically feasible and also provides a direct route to manufacture CO for syngas and hydrocarbon production.
Cycles for splitting CO2 (or H2O) are endothermic and generally require at least one high temperature step to drive the reaction. Concentrating solar power (CSP) and can efficiently supply heat in excess of 800° C. and is potentially suited to operation of thermochemical cycles. Thermochemical cycles are typically categorized by temperature range. High temperature (HT) cycles are those that operate within the limits of most engineering materials and typically involve temperatures between 600 and 1000° C. Ultra-high temperature (UHT) cycles require heat input at temperatures in excess of 1000 and up to 3000° C. Only CSP can be applied to these cycles as materials constraints preclude NE above about 900° C.
Thermochemical cycles have conventionally been studied as potentially a more straightforward, efficient, and lower cost approach to hydrogen production than using electric power to electrolyze water. In the water splitting (WS) scenario, thermochemical cycles employ reactive materials or fluids in a series of chemical reactions that sum to the overall water splitting reactionH2O→H2+½O2 
One class of thermochemical cycles utilizes metal oxides as the internally recycled working material. Fe3O4 is the prototypical working material for these cycles. The overall idealized reaction scheme is:Fe3O4→3FeO+½O2 3FeO+H2O→Fe3O4+H2 
In practice, the temperature required to thermally reduce Fe3O4 to any significant extent is in excess of the melting point of both the oxide reactant and product, while the temperature of the hydrogen producing step is below the melting points. This inherent phase change renders the process unworkable as written. One strategy that has been developed to overcome this problem is to substitute other (A) metals into the Fe3O4 framework that have the effect of lowering the reduction temperature while maintaining the overall spinel structure.
Useful would be a method of splitting CO2 using a similar thermochemical conversion cycle reaction and metal oxides that can be suitable used at operating conditions that favor CO2 splitting.